Silence of the drones: How to quiet that annoying aerial buzz

Drones tend to live up to their name, emitting a low buzz that may be a problem when fleets of delivery drones start operating in urban airspace. But a team from NASA has now developed a way to silence that annoying noise.

The GL-10 is the latest in a series of prototypes from NASA’s Langley Research Center in Hampton, Virginia. It is made from carbon fibre and has a wingspan of 3 metres. But its most unusual feature is the large number of engines, with eight on the wings and another two on the tail. One aim of this is to generate efficient electric propulsion, but the project also solves the problem of aircraft noise&colon; several small motors are quieter than a few large ones.

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“Since we have many propellers, we can operate every motor at slightly different rpm [revolutions per minute],” says project controller Mark Moore. “We have a whole bunch of smaller harmonics and can spread them out across the frequencies. We call this frequency-spectrum spreading, and it’s only possible because we have many propellers and very precise digital control of them.”

This spectrum spreading means that the GL-10 is inaudible when it flies overhead at 30 metres. It can take off vertically, making it ideal as an urban parcel courier for payloads of 5 kilograms or less.

Package delivery is a key objective for drone technology and must be ultra-safe and ultra-quiet, says Moore. “That’s exactly what distributed electric propulsion provides.”

Drone potential

Future plans include larger, manned aircraft using the same technology.

Serge Wich of Liverpool John Moores University, UK, uses drones for conservation work. He says that quieter ones could monitor wildlife without scaring it off. Some birds are sensitive to the noise, and elephants flee from drones – perhaps because they perceive the sound as that of a swarm of bees.

The technology might also be of interest to the military. The noise from the Predator and Reaper drones that patrol in Afghanistan and elsewhere is audible on the ground as an incessant hum, leading locals to call them “machar” (mosquito). NASA’s planned larger aircraft would be as big and fast as the missile-carrying Predator drones.